Field devices, also referred to herein as process devices, are used by the process control and measurement industry for a variety of purposes. Usually such devices have a field-hardened enclosure so that they can be installed outdoors in relatively rugged environments and are able to withstand climatalogical extremes of temperature, humidity, vibration, mechanical shock, etc. These devices also can typically operate on relatively low power. For example, field devices are currently available that receive all of their operating power from a known 4–20 mA loop. These devices are able to not only operate upon the loop but communicate over the loop both with analog signals (actually modulating the 4–20 mA signal) and digitally.
One example of a field device is known as a process variable transmitter. Transmitters sense a process parameter such as pressure, temperature, flow, pH, conductivity, turbidity, etc. and provide a signal over the process communication loop (such as a 4–20 mA loop) that is indicative of the sensed process variable. Process actuators are similar to process variable transmitters in that they generally have a relatively field-hardened enclosure and communicate upon a process control and measurement loop. Process actuators, however, actually affect the process based upon signals received from the loop. Other field devices can include process control modules, process alarm modules, process diagnostic modules, et cetera.
In some process control plants, or in remote measurement stations, it is often useful to provide a transmission of the process variable data by means other than over a copper-wire process communication loop. Usually, a wireless communication is used for such applications. Temporary or add-on process variable monitoring would also benefit from a wireless installation.
For a wireless installation, data is usually transmitted in bursts. During these transmission bursts, relatively significant operating power is required (in excess of 100 milliwatts). During the dwell time between the transmission bursts, the device generally goes into a low-power measurement mode consuming less than 10 milliwatts. In some applications, the unit goes into a sleep mode between measurements. This sleep mode can have an operating power on the order of the microwatts. When process variable measurement is required, or information is required to be transmitted, the unit will then wake up and perform the requisite action.
One pervasive problem for remote, wireless measurement and control applications is providing a reliable, self-contained power source capable of providing adequate power for measurement and transmission. Presently, batteries, or combinations of batteries and solar panels are used by these devices. These approaches have drawbacks because the batteries need changing, or the solar panel sometimes does not receive enough light to adequately charge the back-up batteries. Eventually under these low-light conditions, the measurement and transmission schedule is interrupted due to low batteries. For example, in some geographical locations, low-light conditions can exist, during some seasons, for substantially the entire day.
Providing a power source for wireless remote process installations that does not require either sunlight, or periodically changing batteries would significantly benefit the art.